It seems like when creating a new gallery in the media-frame, the fields in attachment details are not saved automatically unlesss you click away from the field first.
Test:
Add media to gallery
Choose one of the images
Change the "Alt text"
Change to another image
Change back to the first image
Nothing is saved
However this Works:
Add media to gallery
Choose one of the images
Change the "Alt text"
Make the field not active
The alt text field is saved
This only appears to be a problem in the attachment details and not in the gallery settings.
